The invention relates to phosphate based cathode materials for rechargeable batteries with an olivine structure, more in particular non-stoichiometric doped LiMPO4—M=Fe1-xMnx-based cathode materials.
Most commercial rechargeable Lithium batteries use LCO as cathode material. In this document LCO stands for LiCoO2 based cathode materials. However LCO has major drawbacks such as limited safety, where charged batteries might become unsafe, ultimately going to thermal runaway which can result in a severe explosion, and high cost of the cobalt base metal. Substitution of LCO by the cheaper NMC is ongoing, however also NMC shows severe safety issues. NMC is the abbreviation for LiMO2, M=Ni1-x-yMnxCoy based cathode materials.
LCO and NMC belong to the cathode materials with a layered crystal structure. Another crystal structure of Li battery cathode is the spinel structure. Cathode materials with spinel structure are for example LMO or LNMO. LMO stands for LiMn2O4 based cathode materials whereas LNMO is the abbreviation for LiNi0.5Mn1.5O4 based cathode materials. These spinets promise improved safety but show other drawbacks. LMO in practice has a too low capacity and LNMO has a very high charge voltage which makes it very difficult to find a sufficiently stable electrolyte which can operate well within the wide voltage window.
Besides the layered crystal structure cathodes (LCO and NMC) and the spinel structure cathodes (LMO and LNMO), phosphate based cathode materials with olivine structure are also of interest, especially due to their inherently much higher safety. Olivine structured phosphate cathode materials were first proposed by Goodenough in 1996. The Goodenough U.S. Pat. No. 5,910,382 discloses examples for LFP as well as LFMP. LFP stands for LiFePO4 and LFMP stands for LiMPO4—with M=Fe1-xMnx-based cathode materials. An obstacle for the commercialization of olivine crystal structure phosphate cathode materials is the inherently low electronic conductivity. Good electronic contact of the cathode is required because extracting (or re-inserting) of a Li cation requires the simultaneous extraction (or addition) of an electron: LiMPO4→MPO4+Li++e−.
In U.S. Pat. No. 7,285,260, M. Armand and coworkers suggest a method to improve conductivity by carbon coating of olivine. After this disclosure, interest in olivine structure phosphates increased. Commercially most efforts focused on LFP. However—despite of the potentially low cost, high safety and high stability—LFP is still, commercially, a minor cathode material, mostly because LFP has a low energy density. Gravimetric energy density is the product of average voltage and capacity per mass of cathode material. Volumetric energy is the product of average voltage and capacity per volume of cathode material. Despite of a relatively high capacity of about 155-160 mAh/g the energy density (especially the volumetric energy density [Wh/L of cathode]) is insufficient for many applications. This is because of a relatively low crystallographic density (about 3.6 g/cm3) and a relatively low average operation voltage of only 3.3V. For comparison, LiCoO2 has a similar capacity but the average voltage is 4.0V (instead of 3.3V) and the density is 5.05 g/cm3 (compared with 3.6 g/cm3 for LFP).
Already the Goodenough patent teaches that in LFP the transition metal, iron, can be replaced by other transition metals such as manganese. If some Mn replaces Fe then LFMP is obtained, whereas if all Fe is replaced by Mn LMP is formed. LMP stands for LiMnPO4. LMP is of fundamental interest because it has a higher theoretical energy density.
Compared to LFP, LMP has about the same theoretical capacity but a higher average voltage (4.1V versus 3.3V) which promises a significant (24%) increase of the energy density; this effect is however partially offset (−6%) by a lower crystallographic density of LMP (3.4 g/cm3 versus 3.6 g/cm3 for LFP). Up to now, attempts to prepare truly competitive LiMnPO4 failed. The reason for this poor performance is possibly the very low inherent conductivity of LiMnPO4 which, even after carbon coating, prevents achieving a sufficient performance.
Basic properties and issues of LFP, LFMP and LMP are well described for example in “Olivine-type cathodes: Achievements and problems”, Journal of Power Sources 119-121 (2003) 232-238, by Yamada et al. US 2009/0186277 A1 discloses improved LiFePO4 based cathodes by deviating from the Li:M:PO4=1:1:1 stoichiometric ratio. The patent discloses a Li:M (lithium:transition metal ratio) between 1-1.3 and a PO4:M (phosphate to transition metal ratio) range of 1.0-1.14, and the transition metal is selected from Cr, Mn, Fe, Co or Ni. In one embodiment M is chosen as Fe, additionally doped by up to 5% of V, Nb, Ti, Al, Mn, Co, Ni, Mg, and Zr. The examples exclusively refer to M=Fe excluding doping by manganese or other elements. The examples demonstrate an advantage of the Li:M and PO4:Fe ratio being non-stoichiometric. A stoichiometric ratio refers to Li:M:PO4=1.00:1.00:1.00, corresponding to the ideal olivine formula LiFePO4. The examples demonstrate that better LFMP performance can be achieved when choosing a Li:M and PO4:M ratio exceeding 1.0.
In “Reaction Mechanism of the Olivine-Type LixMn0.6Fe0.4PO4, (0<x<1)”, Journal of The Electrochemical Society, 148 (7) A747-A754 (2001), Yamada et al. describe the electrochemical properties of LFMP. When Li is extracted, first a partially delithiated phase is created, the lattice constants change in a single phase manner until all Fe has changed valence state from 2- to 3-valent. After all Fe has reached the 3-valent state further delithiation creates a new phase—fully delithiated LFMP—which coexists with the partially delithiated phase, until all Mn has changed from 2- to 3-valent. The paper gives lattice constants for LFP, LFMP and LMP (see Table 1). In Table 1 the volume is the volume of the full unit-cell, containing 4 formula units of LiMPO4. In the current invention the volume refers to the volume of a single formula unit. Using the data of Table 1 allows calculating an approximate lattice constant for LFMP using Vegard's law (linear change of lattice constants) for stoichiometric LFMP.
TABLE 1Lattice constants of LFP, LFMP and LMPPhasea (Å)b (Å)c (Å)vol (Å3)LFPa1 = 6.008(1)b1 = 10.324(2)c1 = 4.694(1)v1 = 291.1(6)LMPa2 = 6.108(1)b2 = 10.455(2)c2 = 4.750(2)v2 = 303.3(5)LFMP, M = Fe1−xMnxa1(1 − x) + a2(x)b1(1 − x) + b2(x)c1(1 − x) + c2(x)v1(1 − x) + v2(x)
US 2011/0052988 A1 discloses an improved LFMP cathode material. The patent discloses improved performance by additionally doping of M (M=Fe1-xMnx) by up to 10% of Co, Ni, V or Nb. In M the manganese content is 35-60 mol %. The composition of the LFMP olivine phosphate according the patent is not the exact ideal stoichiometric composition (Li:M:PO4=1.00:1.00:1.00) but very near to the stoichiometric composition. The patent discloses a narrow range for Li:M=1.00-1.05, and a narrow PO4:M=1.00-1.020 very near to the stoichiometric value. U.S. Pat. No. 7,858,233 discloses improved performance of LFP, also by deviating from the stoichiometric Li:M:PO4=1.00:1.00:1.00 ratio. Optimum performance is obtained for Fe rich cathodes, where Li:M<1.0 and PO4:M<1.0.
Whereas LCO has high Li diffusion and usually a sufficient electrical conductivity, the Li diffusion rate and electrical conductivity in LFP or LFMP olivine cathode materials is low. Whereas large compact LCO particles (of >20 μm size) can work well as cathode material, LFMP with as similar morphology cannot. LFMP needs to be nano-structured. Nano-structured refers to a morphology, where the Li diffusion path length in the solid is small. In the battery Li diffuses fast in the liquid electrolyte to the nano-particle, and then, in the solid only a short distance in-to or out-from the particle. Because of the short diffusion length good power can be achieved despite of poor diffusivity. Achieving a higher bulk Li diffusion and electrical conductivity allows for good performances with less need to nano-structure the cathode. The prior art does not sufficiently teach how to increase the bulk Li diffusion rate.
The nano-particle itself is usually part of a larger porous agglomerate of smaller sized primary nano-particles. So a high power LFMP cathode material is directly related to a small primary particle size. Besides of microscope investigations the BET surface area is a good tool to estimate the primary particle size. High performance LFMP typically has surface areas exceeding 10 m2/g, whereas surface areas of large particle LCO can be as low as 0.15 m2/g, but still delivering high rate performance LCO. The design of a preferred nano-morphology LFMP is a complex task. The morphology depends on the chemical composition and type of precursors. In many cases milling of precursors before firing is applied, to alter the morphology, but there are limitations. In principle, changing the sintering temperature allows to change the primary particle size, but for LFMP only a relatively small temperature window exists to achieve good electrochemical performance of final cathode products. Practically, temperatures high or low enough to dramatically reduce or increase the BET surface area usually give poor performance.
In the state of the art, there is a lack of efficient tools to alter the nano morphology of LFMP. When designing the optimized nano-morphology typically an increase of BET surface area causes a deterioration of other important parameters. So, nano-structured cathodes often do not pack well, the low pressed density causes a low electrode density, which again reduces the volumetric energy density of the final battery. The electrode density can be estimated by pellet density measurements. There is also a lack of knowledge of how to achieve higher surface areas without significantly deteriorating other properties like electrode density. None of the mentioned prior art improves the olivine structured phosphates sufficiently to make the material truly competitive for commercial mass applications. A further increase of capacity and power is required. Knowledge how to improve the bulk performance by change of composition or doping is needed. Knowledge how a change the composition or how doping can modify and improve the nano-morphology is also not yet sufficient available.
It is an object of the present invention to provide a solution for the problems related to (bulk) electrochemical performance, energy density, nano-morphology, surface area and electrode density.